Polarization-insensitive millimeter-wave directional coupler

ABSTRACT

A polarization insensitive quasi-optical directional coupler comprising three contiguous dielectric layers. Polarization insensitivity of the coupler is realized by selecting the thicknesses and dielectric constants of the layers and the angular orientation of the coupler such that they satisfy predetermined relationships.

625 OR 3174 1 faint/7 Saleh POLARIZATION-INSENSITIVE MILLIMETER-WAVE DIRECTIONAL COUPLER Inventor: Adel Abdel Moneim Saleh, Matawan, NJ.

Assignee: Bell Telephone Laboratories,

Incorporated, Murray Hill, NJ.

Filed: June 21, 1971 Appl. No.: 154,995

US. Cl 350/147, 333/10, 350/152, 350/173 Int. Cl. G021) 27/28 Field of Search 350/147, 152, 157, 350/164, 169, 173; 333/10, 11, 21 A, 24.3

References Cited UNITED STATES PATENTS 1/1971 Refermat et a1. 350/152 11/1964 Lines 333/10 OTHER PUBLICATIONS Otto, Rechnungen zueinem optischen Polarisator nachdem Prinzip der verhinderten Totalreflexion Optik Vol. 29, No. 3 (1969) PP. 246-259 Primary Examiner-John K. Corbin Attorney W. L. Keefauver [5 7] ABSTRACT A polarization insensitive quasi-optical directional coupler comprising three contiguous dielectric layers. Polarization insensitivity of the coupler is realized by selecting the thicknesses and dielectric constants of the layers and the angular orientation of the coupler such that they satisfy predetermined relationships.

5 Claims, 1 Drawing Figure BACKGROUND OF THE INVENTION This invention relates to energy dividing devices and in particular to quasi-optical directional couplers for use at millimeter and submillimeter wavelengths.

In recent years, the need for directional couplers for use at millimeterand submillimeter wavelengths has lead to the development of a variety of couplers which are similar to the couplers which are employed at optical frequencies. These so-called quasi-optical" directional couplers are low-loss devices of simple construction. However, they are also highly polarization sensitive. Due to their polarization sensitivity, quasi-optical directional couplers can only be designed to exhibit a required coupling characteristic for a single polarization. As a result, their use has been confined primarily to systems employing one polarization.

It is therefore a broad object of the present invention to provide a quasi-optical directional coupler which is polarization insensitive.

It is another object of the present invention to provide a polarization coupler which can be used in a microwave system employing more than one polarization.

SUMMARY OF THE INVENTION In accordance with the principles of the present invention, the above-mentioned and other objectives are achieved by employing three contiguous dielectric slabs or layers to form a quasi-optical directional coupler. More particularly, the thicknesses and dielectric constants of the slabs and the orientation of the coupler, with respect to an input wave path, are selected such that the transmitted portion of an incident wave having its electric field polarized in the plane of incidence, i.e., the plane defined by the normal to the surfaces of the layers and the input wave path, has a relative phase and amplitude equal to the relative phase andamplitude of the transmitted portion of an incident wave having its electric field polarized normal to the plane of incidence. Additionally, by choosing the dielectric slabs to be substantially lossless, selection of the aforesaid parameters in the above-mentioned manner also results in the reflected portions of the two waves having equal relative phases and amplitudes. Moreover, since any arbitrarily polarized incidentwave can be resolve into two component waves, one polarized in the plane of incidence and the other polarized normal thereto, it is readily apparent that the transmitted portions of all waves incident upon the coupler will have equal relative phases and amplitudes and, similarly, the reflected portions of all incident waves will likewise have equal relative phases and amplitudes.

In one specific embodiment of the present invention, the two outside dielectric layers of the coupler have equal thicknesses d and equal dielectric constants while the middle layer has different thickness d, and different dielectric constant 5,. In this embodiment d,, 11,, 12,, e, are selected to satisfy predetermined relationships as is more fully disclosed in the equations appearing hereinafter.

DESCRIPTION OF THE DRAWINGS A clearer understanding of the above-mentioned features of the present invention can be obtained by reference to the following detailed description taken in conjunction with the accompanying drawing, in which:

The FIGURE shows a three-layer dielectric coupler in accordance with the principles of the present invention.

DETAILED DESCRIPTION FIG. 1 shows a directional coupler 11 in accordance with the principles of the present invention. Coupler 11 comprises three contiguous dielectric layers 12,13 and 14. Outside layers 12 and 14 have equal dielectric constants c, and equal thicknesses d,. Middle layer 13, on the other hand, has a different dielectric constant e, and a different thickness d,. All three layers, however, are assumed to be substantially lossless.

Coupler 11 is situated along an input wave path 15. The couple is disposed at an angle 0, relative to the latter path, where 0, is the angle between the wave path 15 and the normal to surface 16 (the surface of incidence) of layer 12. Wave energy directed along path 15 and incident upon surface 16 is coupled by coupler 11 along two output paths l7 and 18. The latter path is at an angle 20, relative to path 15 while the former path is parallel thereto. For the purposes of illustration the angle 0 is assumed to be 45. Thus, path 15 and path 18 are at right angles relative to one another.

In operation, two orthogonally polarized plane waves propagating in the plane of FIG. 1 (i.e., the plane of incidence) are directed along wave path 15. One of these waves has an electric field polarized parallel to the plane of incidence, the latter field being represented by the vector E The other wave has its electric field polarized normal to the plane of incidence. The field of this wave is represented by the vector E, which is depicted as indicated in FIG. 1.

The two waves propagate along path 15 and are incident upon coupler 11 at surface 16. Coupler l1 divides each wave into two separate waves, one of which is directed along path 17 and the other along path 18. Those waves directed along path 17 will be referred to as reflected waves while those directed along path 18 will be referred to as transmitted waves.

The reflected wave resulting from the E polarization is designated by the vector E in FIG. 1, while the transmitted wave is designated by vector B As can be seen, both E and E are of the same polarization as their generating wave E Similarly, the reflected and transmitted waves resulting from the E polarization are represented by vectors 13,, and E respectively. Moreover, it is noted that these waves are also polarized in the same direction as their generating wave.

In accordance with the principles of the present invention, it has been recognized that the relative amplitude and phases of E and 5,, and the relative amplitudes and phases of the E and E, can be made equal by selecting the parameters 6,, d d, in accordance with the following relationships:

and

where k is an odd integer, R is the relative portion of an incident wave which is required to be coupled along path 18 and )to is the free space wavelength of the incident energy. 7

Thus, in the present illustrative case, with the aforesaid parameters selected in accordance with the latter relationships, we can write lr/ l ZrI Z and where r and t are defined as the transmission and reflection coefficients of coupler 11 for waves polarized normal to and parallel to the plane of incidence.

Since a wave of any polarization directed along path 15 can be resolved into two wave, one polarized in the plane of incidence and the other normal to the plane of incidence, it is readily apparent that the parameters r and t represent the transmission and reflection coefficients for such as wave also. For example, assume that the polarization E and E instead of representing electric fields of separate waves, are actually the component fields of a single wave having an electric field 12,,

Incidence of this wave upon coupler 11 would result in transmitted and reflected waves E E E E as before. However, from the previous case, we know Thus, the composite reflected and transmitted waves resulting from E, are given as In one embodiment of the invention designed for hybrid operation (i.e., half the energy transmitted and half reflected) at a frequency 100 GHz (i.e., A 3.0 mm) values 'of e, 1.26 and e, 9.4 were found satisfactory. A material having the former value of dielectric constant at a frequency of 100 GHz is Buna S rubber, while a material having the latter value of dielectric constant at 100 GI-Iz is Kearfott Alumina. Other important dimensions of this embodiment are as follows:

Value 0. 73.4 dllko 0.428 d,/A0 0.078

In the embodiment of FIG. 1, it was assumed that the waves incident upon coupler 11 were substantially plane waves and, thus, that coupler 11 was substantially unbounded. However, it should be noted that the principles of the present invention apply equally as well to situations where coupler 11 is bounded as, for example, where it is disposed within a wave guide junction. In such cases, however, the equations derived for the thicknesses and the dielectric constants of the coupler layers would haveto be modified to the degree that guided wave deviates from the plane wave condition.

In all cases, it is understood that the above-described arrangement is merely illustrative of one of the many possible specific embodiments which represent applications of the present invention. Numerous and varied other arrangements can readily be devised in accordance with these principles without departing from the spirit and scope of the invention. For example, instead of selecting the dielectric constants and thicknesses of the first and third dielectric layers to be equal, these parameters can be selected to be different.

What is claimed is:

l. A directional coupler disposed along an input wave path for transmitting and reflecting portions of an incident wave having a wavelength in the millimeter to submillimeter wavelength range propagating therealong comprising:

first, second and third contiguous layers, each of said layers having dielectric properties and being substantially lossless over said range;

said first, second and third layers having dielectric constants and thicknesses such that equal relative portions of said incident wave are transmitted through said layers for all polarizations of said incident wave and such that equal relative portions of said incident wave are reflected by said layers for all polarizations of said incident wave.

2. A directional coupler in accordance with claim 1 wherein said first and third layers have equal dielectric constants e, and equal thicknesses d, and said second layer has a different dielectric constant e, and a different thickness d,.

3. A directional coupler in accordance with claim 2 wherein said parameters e d and (1 are defined by the following relationships:

where 0 is the angle between said input path and the normal to surfaces of said layers, A0 is the free space wave length of said incident wave, k is an odd integer and R is the relative portion of the incident wave which is reflected.

4. Apparatus in accordance with claim 3 wherein 0, is substantially equal to 45.

5. Apparatus comprising, in combination:

an input path,

first and second output paths,

5 6 means for transmitting an electromagnetic wave havsubstantially lossless over said range, said first ing an arbitrary polarization and a wavelength in d thi d l h vi equal thi k d dithe millimeter to submillimeter wavelength range l i cnnstants, and said second layer having along fi'fPP P a thickness and dielectric constant that are difmeans for dividing said transmitted wave into a first 5 ferem from the corresponding parameters of said wave which is directed along said first output path and a second wave which is directed along said sec- 0nd output path, angle and said dielectric constant of said first and said dividing means being disposed along said input third layers, said functions being those for which path at an angle thereto and comprising; 10 said first and second waves have the same polarfirst, second and third contiguous layers, each of ilation 8 a d an itted Wa e.

said layers having dielectric properties and being first and third layers by functions related to said 

1. A directional coupler disposed along an input wave path for transmitting and reflecting portions of an incident wave having a wavelength in the millimeter to submillimeter wavelength range propagating therealong comprising: first, second and third contiguous layers, each of said layers having dielectric properties and being substantially lossless over said range; said first, second and third layers having dielectric constants and thicknesses such that equal relative portions of said incident wave are transmitted through said layers for all polarizations of said incident wave and such that equaL relative portions of said incident wave are reflected by said layers for all polarizations of said incident wave.
 2. A directional coupler in accordance with claim 1 wherein said first and third layers have equal dielectric constants epsilon 1 and equal thicknesses d1 and said second layer has a different dielectric constant epsilon 2 and a different thickness d2.
 3. A directional coupler in accordance with claim 2 wherein said parameters epsilon 1, epsilon 2, d1 and d2 are defined by the following relationship:
 4. Apparatus in accordance with claim 3 wherein theta 1 is substantially equal to 45*.
 5. Apparatus comprising, in combination: an input path, first and second output paths, means for transmitting an electromagnetic wave having an arbitrary polarization and a wavelength in the millimeter to submillimeter wavelength range along said input path, means for dividing said transmitted wave into a first wave which is directed along said first output path and a second wave which is directed along said second output path, said dividing means being disposed along said input path at an angle thereto and comprising: first, second and third contiguous layers, each of said layers having dielectric properties and being substantially lossless over said range, said first and third layers having equal thicknesses and dielectric constants, and said second layer having a thickness and dielectric constant that are different from the corresponding parameters of said first and third layers by functions related to said angle and said dielectric constant of said first and third layers, said functions being those for which said first and second waves have the same polarization as said transmitted wave. 